The present invention relates generally to a pad printing apparatus that employs an engraved printing plate and a deformable ink transfer pad made of silicone rubber or the like, and is particularly concerned with a drive and control system whereby the printing plate and transfer pad are independently driven and controlled. The present invention is also particularly concerned with a closed-reservoir inking assembly for supplying ink to the engraved area of the printing plate.
Typically, with pad transfer printing, an inked image is lifted from the engraved area of an engraved printing plate and is transferred to a surface to be printed by a resilient ink transfer pad, normally made of silicone rubber. The surface characteristics of the silicone rubber are such that the ink easily releases from the pad and adheres to the print receiving surface. The transfer pad typically can elastically deform during printing so that virtually any type of raised or irregular shaped surface can be printed, in addition to flat surfaces.
Various types of automatic printing machines have been developed that employ the pad transfer process. Typically, these machines have an engraved printing plate which is moved between an inking position, where an inking assembly supplies ink to the engraving, and a contact position, where a silicone transfer pad is brought into contact with the inked engraving. The transfer pad typically moves between the printing plate and the surface to be printed. Usually, a common drive system is provided for synchronously moving the printing plate and the transfer pad. The drive system typically includes a linkage for mechanically linking the printing plate and transfer pad so that their movements are slaved or synchronized. Although a common drive system may aid in coordinating the movement of the transfer pad and printing plate, such a slaved system does not allow the pad-printing process to be readily optimized nor allow for many variations in the print cycle. Any adjustments made to the operation of the drive system would necessarily affect the movement of both the printing pad and printing plate. Consequently, conventional drive systems typically will have limited operating ranges. Furthermore, those drive systems typically do not provide a very high degree of precision.
Both open and closed reservoir ink assemblies are known which may be employed in a pad transfer printing apparatus. With an open-reservoir ink assembly, typically, the ink is held in an open trough or reservoir. The engraved area of the printing plate is filled by taking the ink from the trough or reservoir by means of a brush, spreader blade, wire applicator or the like, and applying the ink to the engraved area of the printing plate. A doctor blade or other type of wiping or scraping device is then used to remove excess ink from the plate so that the ink remains only in the grooves or depressions which define the legend to be printed.
With a closed-reservoir ink assembly, the ink reservoir may be inverted and the printing plate positioned beneath the assembly so that the plate holds the ink within the reservoir. As the engraved image of the printing plate moves beneath the reservoir, the ink fills the engraving. Typically, the closed-reservoir ink assembly is provided with a doctoring edge that scrapes excess ink from the plate as the plate moves underneath the ink assembly. In some closed-reservoir ink assemblies the doctoring edge is provided on the inverted reservoir. Furthermore, the assemblies usually include biasing means for applying pressure to the reservoir so that the reservoir is held tightly against the printing plate and the doctoring edge can effectively scrape excess ink from the printing plate. The conventional closed-reservoir ink assemblies, however, typically have a relatively complex structure and therefore can be expensive to manufacture and replace. With one known ink assembly, the ink reservoir, that holds the ink and has the doctoring edge, includes the components for biasing the reservoir against the printing plate. Furthermore, the reservoir includes components for feeding ink into and out of the reservoir and a coupling device for mounting the reservoir to a printing machine. With another known ink assembly, the ink reservoir is mounted into the machine. Should the doctoring edge become worn and either of these reservoirs need to be replaced, the costs associated with manufacturing a new reservoir that includes all the required accessories can be relatively high. Furthermore, due to the number of components and the manner in which the reservoir is typically mounted to the printing machine, the changing of ink type or color can be a time-consuming process involving a high risk of ink spillage as the reservoir is removed, cleaned, filled with ink, and replaced in the machine.